A spindle motor, one of the examples of driving devices, is used for driving disc-shaped recording media such as CD-ROM(R) disc, DVD-ROM(R) disc and DVD-RAM disc. The spindle motor often employs a brush-less DC motor, and is equipped with a turntable, on which a recording medium (an example of rotary units) is mounted, and a driving section for driving the turntable.                The driving section comprises the following elements:                    a motor frame;            a rotor supported rotatably by the motor frame; and            a stator placed in the motor frame such that it surrounds and confronts the rotor.The turntable and the rotor need to synchronize their rotations, so that the turntable is press-fitted or rigidly bonded to the rotary shaft of the rotor.                        
Manufacturing the foregoing spindle motor, i.e. the device used for driving discs, requires improving an accuracy of wobble, i.e. vertical deviation, of the turntable in the vertical direction (“vertical deviation” is a term expressing characteristics). To be more specific, a tolerance of wobble is not more than 30 μm for DVD disc driving devices, and not more than 20 μm for DVD-ROM disc driving devices.
When the spindle motor wobbles during its operation, the recording medium mounted to the motor also wobbles, then a distance between the medium and the head placed closely to the medium changes, so that information cannot be sometimes read or written correctly from or to the medium. This surface wobble, i.e. vertical deviation, often depends on an accuracy of individual components of the motor or an accuracy of mounting the turntable to the rotary shaft. As a result, the vertical deviation due to motor rotation differs in respective spindle motors, and uniform vertical deviations cannot be expected. In order to reduce the vertical deviation as hard as possible, a higher machining accuracy of respective components or a higher assembling accuracy have been pursued.
The spindle motors of this kind have a speed range from approx. 200 to 12000 rpm. CD-ROM disc driving devices and DVD-ROM disc dividing devices, among others, frequently repeat acceleration to a higher rpm and deceleration to a lower rpm, so that abrupt change of torque works, which tends to produce slips between the turntable and the disc.
In order to prevent the slips, a sheet-like or a ring-shaped anti-slipping member has been conventionally bonded on the disc-placeable surface of the turntable. However, this conventional turntable has encountered the following problems: difficulty of manufacturing the anti-slipping member at a uniform thickness, and difficulty of applying the adhesive at a uniform thickness. Those problems cause the vertical deviation of the turntable.
A variety of turntables coated by various anti-slipping films are proposed for solving the forgoing problems. For instance, liquid rubber or liquid thermoplastic resin is printed or sprayed onto the disc-placeable surface of the turntable, then the rubber or the resin is hardened and bonded to the surface by heating or drying. Since this anti-slipping film is printed or sprayed on the turntable, a uniform thickness is obtainable with ease, so that a surface flatness can be easily achieved. This method is thus effective for preventing the vertical deviation of the turntables. This is disclosed in patent document 1, i.e. Japanese Patent Non-examined Publication No. H06-290537.
DVD disc driving devices for driving recordable discs such as DVD-R and DVD-RAM, which have recently become widespread, are required to have an accuracy of vertical deviation as low as not greater than 10 μm, and yet, will be required to have a more strict accuracy. However, the foregoing turntable, of which disc-placeable surface is coated with one of a variety of anti-slipping films, has a limit to keep down the dispersion in accuracy of mounting the turntable to the rotary shaft. It is thus difficult to achieve the more strict accuracy of vertical deviation.
On the other hand, reduction of vertical deviation of a disc placed on a turntable is proposed. The proposal is this: anti-slipping member on a disc-placeable surface is machined such that its surface forms right angles with a rotary shaft of the turntable, thereby reducing the vertical deviation of the disc during the rotation of the turntable. This idea is disclosed, e.g. in patent document 2, i.e. Japanese Patent Non-examined Publication No. H11-25555.
FIG. 5 shows the method disclosed in patent document 2. In FIG. 5, motor 322 is driven to rotate rotor-case 321, and cutting tool 325 is brought from on high into contact with friction member 324 fixed on the disc placeable surface of turntable 323. Cutting tool 325 is moved by a traverse device (not shown) along a direction perpendicular to rotary axis Z of spindle shaft 321b for machining the surface of friction member 324. This method can reduce the vertical deviation of a disc during the rotation of the turntable.
Another method is disclosed is in patent document 3, i.e. Japanese Patent Non-examined Publication No. 2002-238224. Anti-slipping member on a disc-placeable surface of a turntable is polished to be a plane perpendicular to a rotary shaft of a turntable, so that vertical deviation of a disc is reduced.
FIG. 6 shows the method disclosed in patent document 3. In FIG. 6, first of all, rotate turntable (carrier interface) 330 along a first rotating direction, thereby rotating anti-slipping sheet 331 on the disc placeable surface along the first rotating direction. Sheet 331 is formed of a material selected from the group consisting of rubber, silicon, and epoxy. Next, polish sheet 331 with polisher 332 (e.g. round whetstone) rotating along a second rotating direction. During the polishing, polisher 332 moves vertically with respect to the axial direction of shaft 333. As a result, an accuracy of vertical orientation between the surface of anti-slipping sheet 331 and shaft 333 is increased by the polishing, so that the vibration problem due to the vertical deviation of disc can be solved.
According to the methods disclosed in patent documents 2 and 3, a higher accuracy of vertical deviation of turntable's disc-placeable surface is achievable regardless of the accuracy of mounting a turntable to a rotary shaft or the accuracy of coating a disc-placeable surface with anti-slipping film.
The anti-slipping member previously discussed is preferably excellent in anti-slip property (i.e. frictional force), abrasive resistance, and weather resistance, and it is also preferable not to produce blocking between a turntable and a disc. Materials satisfying those requirements have been proposed, for instance, patent document 4, i.e. Japanese Patent Non-examined Publication No. H09-27166 discloses such materials.
As a memory capacity and a reading speed of optical-medium increase, inner disturbing vibrations generated by motor rotations and outer disturbing vibrations, such as vibrations or impacts applied from the outside to the driving devices, have drawn attention as another problem. Those vibrations travel to a disc via a turntable, and generates resonance vibrations on the disc. As a result, an optical pickup cannot read or write signals from/to the disc, so that the device cannot work properly.
In order to solve the foregoing problem, a structure satisfying both the needs simultaneously is proposed, i.e. one need is to convey rotary driving force of a motor enough for disc rotation to the disc and, the other need is to attenuate vibrations generated on the disc and causing abnormality of the device. Such a structure is disclosed in, e.g. patent document 5, i.e. Japanese Patent Non-examined Publication No. H10-302385.
FIG. 7 shows the structure disclosed in patent document 5. Turntable 340 includes a multi-layer structure, i.e. surface elastic layer 341 (anti-slipping section) for conveying the rotary driving force of the motor to a disc and damping vibration elastic layer 342 for damping the vibrations generated in the disc. Layer 341 and layer 342 are bonded with adhesive, when necessary, on the disc-placeable surface of the turntable.
The recordable DVD-disc driving devices have increased their speed, so that the anti-slipping property becomes the more important factor to be achieved. Thus it is crucial to finish anti-slipping faces of disc-placeable surfaces on turntables with a higher accuracy both in roughness and flatness. A rough surface or insufficient flatness will reduce the practical contact area between the anti-slipping face and a disc, so that anti-slipping force is lowered.
However, it is difficult to finish any one of the members previously discussed with a high accuracy by the shaving described in patent document 2 or the polishing described in patent document 3. The members have problems in the processing (machining or polishing), and a hardness of the anti-slipping members cannot be further increased because of maintaining the anti-slipping property. This is another factor to make the processing more difficult.
Although the foregoing conventional processing method of the anti-slipping section of the disc-placeable surface is indeed good enough for achieving a higher accuracy of vertical deviation, the conventional method encounters difficulty of finishing the anti-slipping face with a high accuracy both in a surface roughness and flatness.
Since the anti-slipping member is made of rubber or resin, an edge of the cutting tool tends to gather cut-chips thereon during the shaving, so that a temperature at the edge rapidly rises, thereby shortening the life of the cutting tool. A greater curvature of an edge of the cutting tool and a smaller feeding pitch of the cutting tool are necessary, in general, for the shaving process to achieve a smaller roughness on the surface. This condition makes the edge of the cutting tool tend to gather cut-chips thereon. It is thus difficult to satisfy both of the requirements (i.e. long life of the tool and avoidance of cut-chips).
It is also very difficult to polish soft material such as rubber or resin with a whetstone, because the surface subjected to polishing tends to be rough and produce much abrasive powder. A smaller roughness cannot be expected, and the abrasive powder easily attach to the surface, so that the flatness tends to lower.
In the case of the multi-layer structure including the damping vibration elastic layer which damps vibrations generated on discs, since plural materials are layered, it is more difficult to achieve a higher accuracy with the foregoing shaving or polishing.